1. Field of the Invention
The present invention relates to a technique for electronically controlling a throttle in an internal combustion engine mounted on a car, or the like.
2. Description of the Related Art
In an electronically controlled throttle apparatus for controlling a throttle valve by means of a motor, the quantity of intake air of an engine can be controlled taking into account the condition of the engine obtained from respective sensors as well as the quantity of depression of an accelerator pedal. Accordingly, the electronically controlled throttle apparatus may be used widely for traction control, idling rotational speed control, auto-cruise, lean-burn air-fuel ratio control, etc.
As the control performance required for the use for the aforementioned object, it is said that the response time from the full-open state of a throttle valve to the full-close state of the same is not longer than about 100 ms and the resolving power is about 0.05 degrees. A large number of control systems satisfying these conditions have been discussed in JP-A-63-147947, JP-A-59-188050, JP-A-63-258231 and JP-A-64-77726. For example, JP-A-63-147947 discloses an electronically controlled throttle apparatus using a stepping motor. That is, this JP-A-63-147947 discloses a system in which dither is used to equivalently improve the mechanical resolving power of the stepping motor which is determined depending on the number of poles and gear ratio of the stepping motor.
If the throttle is to be moved very finely upon the assumption of idling rotational speed control, however, hunting may occur in the throttle valve in the vicinity of a target opening in case of depending only on an ordinary feedback control system such as PID control, or the like. This phenomenon is largely affected by nonlinearity caused by static friction in the throttle valve, the motor for driving the throttle valve, and the rotation shaft of reduction gears connecting the throttle valve and the motor to each other. If a large torque is applied to a stationary valve so as to move the valve, the opening exceeds the target value so that a large torque must be applied again reversely.
In such a case, dither can be used to give fine vibration onto the throttle valve to thereby suppress the effect of static friction. It is not preferable to give vibration always to the throttle valve from the point of view of durability. When the motor is a DC motor with brushes, abrasion of the brush causes a problem. When the throttle valve opening sensor is a contact type potentiometer, there is a possibility that no sensor output is obtained because of the abrasion of the contact surface of the potentiometer.